The domino is the unique free (and one-sided) 2-polyomino consisting of two equal squares connected along a complete polygon edge. There are two fixed dominoes.

Culin, S. "Kol-hpai, Bone Tablets--Dominoes." §81 in Games of the Orient: Korea, China, Japan. Rutland, VT: Charles E. Tuttle, pp. 102-103, 1965.

Gardner, M. "Polyominoes." Ch. 13 in Hexaflexagons and Other Mathematical Diversions: The First Scientific American Book of Puzzles and Games. New York: Simon and Schuster, pp. 124-140, 1959.

Kraitchik, M. "Dominoes." §12.1.22 in Mathematical Recreations. New York: W. W. Norton, pp. 298-302, 1942.

Lei, A. "Domino." http://www.cs.ust.hk/~philipl/omino/domino.html

Madachy, J. S. "Domino Recreations." Madachy's Mathematical Recreations. New York: Dover, pp. 209-219, 1979.

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Weisstein, Eric W. "Domino." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Domino.html